1. Field of the Invention
This invention relates to an electric resistor having a large negative temperature coefficient of resistance, that is, an NTC thermistor (negative temperature coefficient thermistor). More particularly, it relates to a method for making a carbide thin film thermistor.
2. Description of the Prior Art
In the manufacture of known NTC thermistors, it is the general practice to use materials which are obtained by sintering mixtures of various oxides of metals such as Fe, Ni, Co, Mn and the like at high temperatures. Since the B constant of the oxides of the just-mentioned type is usually in the range of 4000.degree.-6000.degree. K., a thermistor made of these oxides has the advantage that the rate of variation in resistance is large as the temperature varies by unit degree but has the disadvantage in that it is not possible to detect by a single element a temperature variation over a relatively wide range, say, from room temperature to 400.degree. C. This is attributed to the fact that thermistors using such metal oxides having a large B constant are so varied in resistance as to be outside a useful range (about 1-100K.OMEGA.) when subjected to such a wide range of temperature as mentioned above. Accordingly, when known NTC thermistors are used to control the temperature of for example cooking or combustion devices, undesirably two types of thermistors for low and high temperatures have to be employed since a necessary control range of temperature for the above purpose is usually from room temperature to 400.degree. C. This also involves further disadvantages such as a complicated circuit arrangement, a high production cost, and a lowering in reliability.
The thermistor of the type mentioned above is ordinarily employed in an atmospheric environment under which it is very stable. In this connection, however, when the thermistor is utilized under severe conditions, such as in cooking or combustion devices, contaminated with or exposed to vapors or fine droplets of seasonings such as various sauces, soys, oils, salt solutions, water, or exhaust gases resulting from high temperature combustion, the metal oxides tend to be reduced, with the attendant variation in their characteristic properties.
Aside from sintered materials comprising mixed oxides, there are also known thermistor elements or chips which include an insulating substrate, having electrode films and a temperature sensitive resistor film formed on one surface of the substrate. Typical of the temperature sensitive resistor films are vacuum evaporated films and sputtered films such as of Ge, the afore-mentioned metal oxides, SiC, and the like. However, the resistor films of Ge or the oxide mixtures have disadvantages similar to those of the sintered materials comprising oxide mixtures. Though the SiC resistor film has excellent stability to heat and an excellent resistance-temperature characteristic suitable for detecting a wide range of temperatures, the film usually has to be formed by the sputtering method, involving several problems described below.
Sputtering techniques have been widely used to form various materials such as, for example, conductive materials, dielectric materials, and semiconductive materials into thin films for making electric parts such as resistors, capacitors and the like.
With resistors, the fundamental characteristics of the resistors depend on the type of resistive materials, and various target materials are used depending on the purpose and end use. As a matter of course, the process of making the resistor film is also an important factor which has a great influence on the electrical properties of the film, resistive properties, deposition thickness, specific resistance and the like. Among the various parameters of the sputtering technique, the most important ones are sputtering gas pressure, substrate temperature, sputtering power, sputtering time and purity of sputtering gas. The sputtering technique makes use of a phenomenon where ionized gas molecules are accelerated by an electric field and caused to collision with a target whereupon the target material is emitted in the form of atoms or molecules. In the sputtering procedure, it is conventional to use Ar gas at 10.sup.-1 -10.sup.-3 Torr. pressure having a high purity of about 99.9999%. The substrate is heated and held at a suitable temperature to ensure a good adherence to the deposited film. Choice of the temperature is made in consideration of the temperature necessary for removing water or organic matter from the surface of the substrate (e.g. above 100.degree. C.), the temperature at which the thermal expansion coefficients of the substrate and the film come close to each other, and the temperature at which the sputtered material is not decomposed. The sputtering power contributes in direct proportionality to the thickness growth per unit time and is generally in the range of 1-5 KW, because when the incident ion energy is too great, the characteristics of the resulting film vary due to excessive rise in the surface temperature. The sputtering time is dependent on the desired thickness of the film.
Carbide resistor films have been heretofore formed by the sputtering technique as follows. When sintered SiC is used, for example, as a target material, the SiC thin resistor film is formed on a substrate of a selected temperature under such conditions as, for example, an rf power of 2 KW (frequency: 13.56 MHz), sputtering gas pressure on the order of 10.sup.-2 Torr., Ar sputtering gas (purity: 99.9999%), and sputtering time of 4-8 hrs. However the SiC resistor film obtained by such a method is disadvantageous in that its specific resistance or sheet resistance is great and that the specific resistance and the B constant are both scattered to an extent. Especially, the scattering in values of the specific resistance or sheet resistance and the B constant is a critical disadvantage in the manufacture of a thermistor using a SiC resistor film as a temperature sensitive resistor. In addition, in cases where conditions other than the sputtering time are held constant, the film thickness increases in proportion to the sputtering time but the sheet resistance is not necessarily inversely proportional to the thickness. Presumably, this is because the B constant varies depending on the sputtering time. This leads to a further disadvantage that where a SiC resistor film of a small sheet resistance value, particularly in a temperature range near room temperature is needed, the specific resistance gently decreases in relation to a film thickness, so that the sputtering time has to be made very long. The very long sputtering operation undesirably requires great amounts of materials, energy, and operation time and results in a high production cost.
Further, resistor films of carbides such as SiC have an additional disadvantage in that they are very high both in hardness and melting point, so that the film is hard to trim in a desired pattern in order to finely adjust its resistance value. That is, when the film is required to have an accurate resistance value, it is the usual practice to finely trim the electrode film or the resistor film in a desired manner by the sand blast trimming method or the laser trimming method to adjust its resistance to a predetermined level. In this connection, however, the former method is not applicable to a very hard film since such a film is not removed by fine particles of SiC, Al.sub.2 O.sub.3 or the like which are blown from a nozzle under pressure. The latter method is not suitable for application to a high melting film such as SiC since the film is hardly evaporated by application of the laser beam.